In discussion of the 5th generation 5-Generation, 5G for short) mobile communications standard, low latency is one of key indicators. This means that a transmission time interval (Transmission Time Interval, TTI for short) needs to be reduced, so as to accelerate hybrid automatic repeat request—acknowledgment (Hybrid Automatic Repeat request acknowledgment, HARQ-ACK) feedback and retransmission of data, and reduce end-to-end data latency.
In a conventional Universal Mobile Telecommunications System (Universal Mobile Telecommunications System, UMTS for short) or Long Term Evolution (Long Term Evolution, LTE for short) system, one TTI occupies several symbols, and the several symbols are used to send one data block. At a receive end, a demodulation and decoding operation is performed only when all symbols of one TTI are received, and corresponding acknowledgment (Acknowledgment, ACK for short) feedback and possible retransmission also need to be performed at a complete TTI. Using UMTS as an example, after receiving a high speed physical downlink shared channel (High Speed Physical Downlink Shared Channel, HS-PDSCH for short), user equipment (User Equipment, UE for short) performs demodulation and decoding and sends a feedback HARQ-ACK to a base station; the base station determines whether to perform retransmission or new transmission only after receiving the HARQ-ACK sent by the UE. For example, an earliest retransmission moment corresponding to a subframe 0 is a subframe 6. This is mainly restricted by a demodulation and decoding time of a user, a TTI length, and a subframe structure. Timing in LTE is similar.
Currently, a TTI in UMTS is 2 ms, and a TTI in LTE is 1 ms. A 5G low latency service is expected to reach a latency of less than 1 ms. This means that a TTI length needs to be decreased, to reach the target. For coexistence with the conventional LTE system, a conventional LTE frame structure can be used, and each original orthogonal frequency division multiplexing (Orthogonal Frequency Division Multiplexing, OFDM for short) symbol is used as a TTI. For example, in a short-TTI system, each symbol is only 1/14 ms, and a round trip time (Round-Trip Time, RTT) is eight symbols (about 0.6 ms). This can reach the low-latency target with a latency of less than 1 ms. In another short-TTI system design, a time length of each symbol in a TTI may be reduced, and each TTI still includes several symbols. In this case, only a time length of the TTI needs to be controlled to meet a latency requirement.
An advantage of a short-TTI system is low latency because each TTI has a quite short time length, and HARQ-ACK and retransmission is quite fast. A disadvantage lies in that control overheads are relatively high. Especially for a single-symbol TTI, data sending for each symbol needs a corresponding pilot, scheduling information, and HARQ-ACK information, and in downlink, corresponding feedback information of channel state information (Channel State Information, CSI) is also required. Therefore, a large amount of control information needs to be delivered.